The invention relates generally to steam turbines with drum rotors and more specifically to retention arrangements for buckets of steam turbines with drum rotors.
The current practice for radial loading of steam turbine reaction style buckets involves inserting a dovetailed portion for each bucket into a retaining groove in the steam turbine drum rotor, inserting a loading pin in a tightly controlled radial gap between a base of the retaining groove and a cutout in the bottom of the dovetailed portion, and then hammering the pin into the radial gap such that the pin deforms in the rotor radial direction and loads the bucket radially against a hook in the retaining groove.
For each bucket, there is a loading pin and each loading pin must be hammered manually until the bucket does not move in the rotor groove. This hammering operation, however, introduces an opportunity to damage the bucket as well as the rotor. As an example, for a 30-stage high pressure steam turbine approximately 2600 loading pins must be manually hammered to fasten the buckets radially in place.
FIG. 1 illustrates a representation of a portion of a steam turbine 5 including a drum rotor 24 mounting a plurality of circumferentially spaced buckets 10 about the periphery of the drum rotor 24, the drum rotor having an axis of rotation 7. The buckets 10 are arrayed in circumferentially extending female dovetail slots 26 in the drum rotor 24. A steam turbine casing 29 surrounds the drum rotor 24 and includes a plurality of nozzle segments 21 spaced circumferentially one from the other located in female dovetailed slots 27 in the casing 29.
FIG. 2 illustrates a first arrangement for maintaining radial loading on the root of the bucket being retained in the drum rotor. With reference to FIG. 2, a turbine bucket 10 includes an airfoil portion 12 and a root or base portion 14 that is configured as a male dovetail 16. The male dovetail includes radially outer and inner projections or hooks 18, 20 radially spaced by a narrow neck 22.
The drum rotor 24 is formed with an annular bucket retaining groove configured as a female dovetail slot 26 about the periphery of the wheel with a radially outer wide groove portion 28 for receiving the outer male projection 18, a radially inner wide groove portion 30 for receiving the inner male projection 20, and an intermediate narrow groove portion 32 for receiving the narrow neck 22. An undersurface 33 of the narrow groove portion 32 forms a so-called “hook” that is engaged by the inner projection 20 on the male dovetail 16.
A semicircular retaining groove 35 extends across undersurface 40 of male dovetail 16. When each bucket is loaded into female dovetail slot 26 about the periphery of the wheel, a solid semicircular pin 37 is manually hammered into the semicircular retaining groove 35 to bias the bucket in a radially outward direction, loading the bucket radially against the hook 33.
FIG. 3 illustrates an alternative arrangement for loading the bucket radially against hook 33. The alternative arrangement was described in U.S. Pat. No. 6,761,538 by Fitts et al. Corresponding parts of the female dovetail slot and male dovetail are similarly numbered as in FIG. 2. Within the base 34 of the female dovetail slot, there is formed an annular spring retaining groove 36 that extends completely about the periphery of the wheel. The groove itself extends substantially 180.degrees when viewed in cross-section (as in FIG. 1). A loading spring segment 38 is shown within the groove 36, radially interposed between the base 34 of the dovetail slot and the radially inner face 40 of the bucket dovetail. As indicated above, more than one groove 36 may be used, depending on the required radial loading on the buckets. The spring segment 38 biases the bucket in a radially outward direction, loading the bucket radially against the hook 33.
Traditional methods, and variations thereof, have been to essentially push the bucket outwards, radially, to ensure tight contact between the bucket load surface 33 and the rotor mating surface 39.
Accordingly, there is a need for an improved radial loading technique that provides parts reduction, rotor assembly time reduction, and consistent radial loading of the buckets against the rotor groove hook without danger of damage to the buckets and/or rotor.